The NIH mouse potency test for rabies vaccine is a highly variable assay system. We have been attempting to replace this test with a more reliable in vitro system such as the Single Radial Immunodiffusion Test (SRID) and the antibody binding test. The SRID test seems to be the most promising candidate at this time. We currently have two licensed manufacturers of rabies vaccine in the U.S. One product is adsorbed on aluminum phosphate and one is unabsorbed. We are testing each lot of the latter product by SRID and approximately 60% of the lots by the NIH test. The manufacturer is also testing each lot by both methods and reporting these results to us so that a statistical comparison of the two methods can be performed. The adsorbed vaccine cannot be tested by SRID, therefore, the antibody-binding test (using BHK-21 cells) and FITC-conjugated antirabies serum is used for this product in parallel with the NIH test. We also participated in two separate WHO collaborative studies comparing the above test methods in the replacement of the International Standard Rabies Vaccine. A report for the first study will be presented to the WHO Expert Committee on Biological Standardization in the fall of 1991. The second study was initiated in July 1990 and a draft report has just been circulated for comment.